The principle of laser operation is stimulated emission of energy. When an electron is in an upper (i.e., excited) energy level of the laser material and a lightwave of precisely the wavelength corresponding to the energy level difference between the unexcited and excited states strikes the electron, the light stimulates the electron to move down to the lower level and emit a photon. This photon is emitted in precisely the same direction and phase with that of the incident photon. Thus, a light wave is established in the laser material, and if it can be made to travel back and forth through the laser material (also known as the gain media) it will retain its frequency and grow in amplitude as it stimulates photon emissions. This positive feedback mechanism is accomplished by a mirror placed at each end of the lasing material to reflect the traveling wave back through the laser material The rear mirror is fully reflecting, and the front mirror is partially reflecting and partially transmitting at the laser wavelength. Light reflected back and forth from the front and rear mirrors serves as positive feedback to sustain oscillation, and the light transmitted through the front mirror is the laser output light. The two mirrors are parallel and form an optical cavity that can be tuned by varying the spacing between them. The laser can operate only at those wavelengths for which a standing-wave pattern can be set up in the cavity, i.e., for which the length of the cavity is an integral number of half wavelengths.
At room temperature, the electron ground state is almost entirely occupied, and the upper energy levels are essentially unoccupied. When the upper energy level has a greater electron population than the lower level, a population inversion exists. This inverted population can support lasing since a traveling wave of the proper frequency stimulates downward transitions of the electrons with the associated energy release. The process of exciting the laser material to raise the electrons to an excited state, i.e., producing a population inversion, is referred to as pumping. Pumping can be accomplished optically with a flash lamp driven at a high frequency, by an electric discharge, by a chemical reaction, or in the case of a semiconductor laser, by injecting electrons into the upper energy level with an electric current. When a sufficient number of electrons are in an excited state, the laser energy can be released by allowing the traveling wave to exit the laser cavity.
In a typical YAG laser, the laser material comprises yttrium, aluminum, and garnet surrounded by a helical flash lamp. The front and rear mirrors are spaced apart from the laser material on the same longitudinal axis. The flash lamp is driven on and off to excite the electrons in the laser material, moving them to a higher energy level. The typical output of an optical laser consists of a series of spikes occurring during the time the laser is pumped. Spikes are created because the inverted population is being alternatively built up and depleted.
Q-switching (Q-spoiling) is a laser control technique for obtaining all the energy in a single spike of very high peak power or spreading the energy over a series of laser output pulses. As an example of the former, a typical laser generates approximately 100 mJ over an interval of 100 microseconds for an average peak power of 1000 W. The same laser can be Q-switched to emit 80 mJ in a single 10 nanosecond pulse for peak power of 8 MW. Prior art Q-switching is accomplished by alternately completely inhibiting and allowing a laser output signal. As a simple example, the Q-switch could be a mechanical shutter between the laser material and the front mirror. While the shutter is open, a laser output signal is produced. While the shutter is closed, the pumping process continues, but no laser light is emitted from the front mirror. When the shutter is closed for a relatively long time and then opened, a giant laser pulse is emitted. Further opening and closing of the shutter then produces a series of lower-power (relative to the first pulse) laser output pulses. The shutter can be driven by light-request pulses, wherein each light-request pulse opens the shutter allowing laser light to escape, but the shutter has obvious speed/frequency limitations due to its mechanical nature. As the laser pumping frequency increases, the amount of energy in the first pulse also increases relative to the steady-state laser pulse energy.
A high-peak-power pulse from a Q-switched laser is useful in optical ranging and communications and in producing nonlinear effects in materials. The series of output pulses from a Q-switched laser are useful in material heating and material removal, i.e., material scribing where a first material is deposited over a second material and each laser pulse etches away a small portion of the first material (scribes it) to produce a predetermined pattern on the second material.
One popular type of Q-switch is an acousto-optic version. In this version, a transducer constructed of lithium niobate is mounted on acousto-optic material. The transducer is driven by an RF signal that creates a grating pattern in the acousto-optic material. Laser light also impinges on the acousto-optic material such that it is perpendicular to the direction of the acoustic wave (created by the RF signal) through the acoustic-optic material. The spacing of the grating pattern depends on the frequency of the RF signal, and when the spacing reaches a certain minimum, the laser light will be deflected by the grating pattern and not passed through the acousto-optic material. Thus, there is a certain minimum RF frequency that must be used to cut-off or spoil the laser output signal. Pulses, typically referred to as light-request pulses, are provided as an input to a driver stage that controls the acousto-optic Q-switch. The pulses modulate an internally generated RF signal. During each request pulse the RF signal goes to zero; thus, the grating pattern disappears and an output signal is emitted by the laser. Between light-request pulses, when the modulated RF signal attains its normal peak-to-peak amplitude, the Q-switch is activated and there is no output signal from the laser.
In prior art acousto-optic Q-switches the driver stage responds to the request pulses by bringing the RF signal to zero during each request pulse and allowing the RF signal to attain its normal peak-to-peak amplitude between request pulses. As discussed above, this on/off control scheme creates a giant first laser output pulse when the laser has not produced an output signal for a relatively long time. This "off" time is relative with respect to the frequency of laser operation. When the Q-switched laser is to be used in its pulsed mode (where each pulse does some useful work, for instance in material scribing or heating as discussed above) the giant first pulse must be prevented from reaching the working material to avoid damaging it. Further, increasing the laser frequency to increase the efficiency of the operation is counterproductive because as the laser frequency increases the ratio of first pulse energy compared to the steady-state energy also increases. As the frequency increases and additional energy is contained in the first pulse, there may also be insufficient energy in the succeeding pulses to perform useful work. (Laser frequency is increased by increasing the amount of energy that is input to the laser to create the population inversion.)
Other types of Q-switches are well known in the art including an electro-optic Q-switch, a magneto-optic Q-switch, and a saturable organic-dye absorber Q-switch. The disadvantages associated with the production of a giant first pulse are associated with each of these Q-switch types.
Thus, it would be advantageous to limit the energy contained in the first laser pulse so that it is approximately the same as the energy in succeeding laser pulses. Accomplishing this objective also allows laser operation at a higher frequency to provide more efficient use of the laser in material heating, material scribing, or any of the other uses associated with a pulsed laser.
U.S. Pat. No. 4,675,872 (1987) to Popek et al. discloses a pumped laser system having a Q-switch which controls the laser output. In order to eliminate the giant pulse from the Q-switch output, adjustable Q-switch spoiling is provided at the beginning of each sequence of output pulses.
U.S. Pat. No. 5,226,051 (1993) to Chan et al. discloses a method and apparatus to control the power delivered to the laser pump in order to limit the maximum energy of any single laser pulse output as a result of Q-switching. The laser pump is driven at full strength for a selected initial refresh time and then shut off. If a Q-switch trigger signal is received during the shut-off period, then only the reduced energy stored in the laser is released. An algorithm reduces the power to a lower level sufficient to maintain the net energy stored in the laser at a predetermined value if the elapsed time reaches or exceeds the refresh time. This is the closest known prior art. The system cannot be used for widely varying time intervals between the pulses. The system cannot control the energy of each pulse; rather, it can only limit the maximum energy of each pulse.
U.S. Pat. No. 5,016,251 (1991) to D'Arcy discloses a Q-switch driver for controlling the laser output in a long-pulse mode only. A means to calculate a timing window for triggering the Q-switch is provided, wherein the Q-switch remains open during the window period. The device is limited to a narrow range of beam intensities and not appropriate for general purposes.
U.S. Pat. No. 5,018,152 (1991) to Linne et al. discloses a Q-switched laser having a detector to sense the population inversion using the fluorescence signal and then via a feedback loop to generate the energy control signal. As the switching rate of the Q-switch is varied, the energy control signal via the feedback signal is varied to ensure a constant level of population inversion between each pulse. The device attempts to provide a pre-specified pulse energy for each pulse independent of the switching rate of the Q-switch. Drawbacks to this approach include the requirement for extra devices and circuits to measure the energy and produce the feedback signal. Also, calibration problems add costs to the mass production of such devices.
All of the above approaches allow the gain media to be excited to a rather uncontrollable state and then to attempt to release small controllable bursts of energy. Only Chan attempts to control and maintain the gain media in an energy state which is useful for producing a steady energy level output.
The present invention recognizes that known gain media materials including Nd:YAG, Nd:YLF, Nd:YVO4 have corresponding fluorescent lifetimes. These lifetimes are respectively 230 microseconds, 480 microseconds, and 90 microseconds. The present invention models the diode current driver to pulse the gain media with a pulse width as short as 20% of the fluorescent lifetime. The result is controlling the gain media in a useful range of excitation at all times. In order to provide a steady energy laser output, the Q-switch is synchronized to open at the end of each drive pulse to the diode current driver.